Ti₃C₂T _x MXene-Enhanced Paper-Based Ion-Selective Electrodes for Sweat Sodium Monitoring

Electrolyte balance plays a fundamental role in maintaining physiological homeostasis, as ionic fluctuations regulate cellular excitability, osmotic gradients, and neuromuscular signaling. Among electrolytes, sodium (Na⁺) is a critical biomarker for hydration status and several pathological conditions, including dehydration and cystic fibrosis. Here, this is the first report of Ti₃C₂T _x MXene integrated as a solid-contact transducer on a paper-based screen-printed electrode (MPSPE) for ion-selective sensing of Na⁺ in sweat. The platform combines the sustainability and intrinsic capillarity of paper substrates with the exceptional electrical conductivity and surface reactivity of Ti₃C₂T _x MXene. Prior to MXene deposition, oxygen plasma treatment was applied to improve surface hydrophilicity and promote homogeneous film formation. The optimized configuration, based on office paper modified with 3 mg mL^-1 MXene, exhibited a near-Nernstian slope of 0.0637 V/decade and an excellent linear correlation (R ² = 0.98) over the tested Na⁺ range (1-150 mM). The calculated limit of detection (LOD) and limit of quantification (LOQ) were 0.35 mM and 1.16 mM, respectively, while the coefficient of variation (%CV) was found to be 7.2%, confirming outstanding repeatability. The electrode demonstrated high selectivity toward Na⁺ over K⁺, Mg²⁺, and Ca²⁺ (log K^pot _Na,j < -0.5). Validation with real sweat samples revealed an excellent correlation with the liquid chromatography-tandem mass spectrometry (LC-MS/MS) reference method (R ² = 0.96, mean correlation ≈ 99%). These results highlight the potential of MXene-modified paper electrodes as a low-cost, disposable, and environmentally friendly analytical platform for point-of-care electrolyte monitoring.